This invention relates in general to electronic control units for anti-lock brake systems and in particular to a current feedback circuit and a method for monitoring operation of a solenoid valve in an anti-lock brake system.
An Anti-lock Brake System (ABS) is often included as standard or is optional equipment on new vehicles. When actuated, the ABS is operative to control the operation of some or all of the vehicle wheel brakes. A typical ABS includes a plurality of solenoid valves mounted within a control valve body and connected to the vehicle hydraulic brake system. Usually, a separate hydraulic source, such as a motor driven pump, is included in the ABS for reapplying hydraulic pressure to the controlled wheels during an ABS braking cycle. An ABS further includes an electronic control module which is electrically connected to the pump motor, a plurality of solenoid coils associated with the solenoid valves, and wheel speed sensors for monitoring the speed and deceleration of the controlled wheels. The electronic control module includes a microprocessor which is programmed to operate the ABS. The electronic control module is typically mounted upon the control valve body. The assembled valve body, motor and control module form a compact unit which is often referred to as an ABS control valve.
During vehicle operation, the ABS control module continuously receives wheel speed signals from the wheel speed sensors. The microprocessor in the control module monitors the wheel speed signals for potential wheel lock-up conditions. When the vehicle brakes are applied and the microprocessor senses an impending wheel lock-up condition, the microprocessor is operative to actuate the pump motor and selectively operate the solenoid valves in the control valve to cyclically relieve and reapply hydraulic pressure to the controlled wheel brakes. The hydraulic pressure applied to the controlled brakes is adjusted by the operation of the solenoid valves to limit wheel slippage to a safe level while continuing to produce adequate brake torque to decelerate the vehicle.
Proper operation of the solenoid valves in the ABS is not only required for controlling the braking of the vehicle, but safety aspects require disabling the ABS should a valve malfunction. Accordingly, it is known to include pressure sensors in the hydraulic brake lines to monitor operation of the solenoid valves. The pressure sensors are connected to the microprocessor in the ABS electronic control module. After the microprocessor causes a change in the operating condition of one of the solenoid valves, the microprocessor monitors the brake line pressure for a change in the hydraulic pressure resulting from the operation of the valve. If the change is not detected within a predetermined time period, the microprocessor deactivates the ABS and illuminates a warning light to alert the vehicle operator of an ABS malfunction.